Can PVC Conduit Cause Overheating?

bshanleyJuly 1, 2011

I'm installing an inlet from outside my home for my transfer switch. I used 10/3 building wire, with a ground. For neatness, I used 1/2" PVC Schedule 40 conduit in two places, one about 5', one about 3'. It was a pretty tight fit getting the wire through. Now I'm wondering, will the tight fit cause overheating? I haven't tried it out yet. Thanks in advance.

Your direct question is about overheating. No problem in your case. You did install NM cable in the conduit. If the conduit is in a "dry" location, the cable will not suffer from moisture damage. If these were individual conductors in the conduit, the installation would meet NEC specs for "fill". Based on the information you furnished, your installation will function satisfactorily.
Others will be along to debate the use of the conduit with the NM cable.

Conduit or not, you have a problem if you installed NM (non-metallic) cable, since it isn't rated for wet locations. NM cable will rapidly fall apart under wet conditions (and the inside of conduit is particularly wet). You should have used UF cable or alternatively THWN individual wires inside conduit.

Hi Kurto: Thanks for your advice. Let me say a little more, to make sure we're on the same page. I ran 40' of wire. 39' were inside the house, and all the conduit was inside the house. The final foot went through the block (10"), and the last little bit was inside a UL rated box, on the outside wall. I sealed the gap between the house and box on the top and two sides with Silicone II. Does it still sound as bad? How is it materially different than the wiring that runs to the light at my front door, which is not really wired as well? Thanks again - I'm not disputing your advice, just want to learn.

I don't think it's a violation. The box is not a raceway even if it is in a wet location (though it does need to be rated for such the wires inside do not) and the raceway (conduit) is in not in a wet location.

The conduit provided to protect the NM from physical damage here is acceptable.